This invention pertains to drainage systems for flexible floating covers covering large wastewater reservoirs, and particularly it relates to a drainage system for accumulating rainwater on a membrane cover in a controlled manner and for draining excess rainwater into the wastewater reservoir.
Membrane covers are mounted over settling ponds, clarifiers and other wastewater reservoirs to contain and collect fermentation gases of mill effluent for example. The maintenance of a membrane cover floating on a large wastewater reservoir represents certain difficulties in that the cover is exposed to the elements and to the movement of the liquid under the cover. For example, a slight accumulation of rainwater over a cover tends to create puddles, mounds and relaxed segments at random locations on the cover. These puddles, mounds and especially the relaxed segments catch the wind and promote waves along the cover and into the liquid under the cover. The movement of liquid under the cover causes tangential stresses and constant movement in the membrane cover itself. These movements and stresses could cause fatigue, localized elongation and rupture of the cover. The formation of mounds, puddles and relaxed segments on a membrane cover is amplified where the content of the covered reservoir contains gases or is able to generate gases that tend to create gas pockets at the surface of the wastewater under the membrane cover.
Although the formation of rainwater puddles and mounds over a floating cover is a natural phenomenon that will remain an inherent characteristic of a flexible membrane cover, it will be appreciated from the following disclosure that there are numerous advantages which can be derived from an accumulation of rainwater over a floating membrane cover when the accumulation and drainage of rainwater are effected in a controlled manner.
It is believed that the prior art is short on suggestion with regards to a drainage system to advantageously control the accumulation of rainwater on a membrane cover. Examples of the drainage systems of the prior art for floating covers are described in the following documents:
U.S. Pat. No. 2,531,898, issued on Nov. 28, 1950 to R. C. Ulm, discloses a floating roof with a flexible deck and a central weight mounted on the deck. The central weight causes the deck to curve downward such that rainwater flows downward toward the central weight. Rainwater is evacuated through a drain at the centre of the weight and through a hose extending from the drain.
U.S. Pat. No. 4,672,691, issued on Jun. 16, 1987 to DeGarie et al. discloses a flexible membrane cover having weight lines thereon to promote the accumulation of rainwater under the weight lines. Rainwater is evacuated by evaporation or by the use of sump pumps placed directly on the membrane cover.
U.S. Pat. No. 5,946,743, issued on Sept. 7, 1999 to I. S. Hashmi, discloses a pool cover having a drain hole at the centre thereof. A flexible conduit extending from the drain pipe carries rainwater outside the pool.
Although the drainage systems of the prior art deserve undeniable merit, there is no known prior art that discloses, teaches or suggests a drainage system to control the accumulation of rainwater on a membrane cover such that the cover is less susceptible of being exposed to destructive stresses from wind-induced liquid movement under the cover.
In the present invention, there is provided a membrane cover and a drainage system therefor whereby rainwater is quickly accumulated during the early stages of a rainstorm to provide a water ballast on the membrane cover to keep the cover taut and to limit the formation of randomly spaced puddles, mounds and relaxed segments that can deform the cover and create destructive stresses in the cover structure.
In a first aspect of the present invention there is provided a wastewater reservoir having a membrane cover mounted thereon. The membrane cover has a flexible buoyant impermeable membrane affixed to the perimeter walls of the reservoir and loosely laid over the wastewater level inside the reservoir. An array of weight lines is anchored to the perimeter walls of the reservoir and is loosely laid over the impermeable membrane. The array of weight lines comprises a longitudinal weight line and a plurality of spaced-apart lateral weight lines extending perpendicularly from the longitudinal weight line on both sides of the longitudinal weight line. An array of drains is also provided. Each drain is disposed along one of the lateral weight lines and extends through the impermeable membrane. The membrane cover according to the present invention also comprises an array of troughs formed therein under the array of weight lines. The troughs are connected to each other. A water ballast is contained within the connected troughs and a level of water in this water ballast is controlled by the drains. The water ballast defines a herringbone pattern extending over a major portion of the cover.
A first advantage of the membrane cover having the water ballast thereon is that is the cover is kept taut and stable during a rainstorm and a surface thereof exposed to uplifting wind forces is minimized.
In another aspect of the present invention, the membrane cover comprises a flexible buoyant layer affixed to the perimeter walls of the reservoir and loosely laid over the wastewater level inside the reservoir. The impermeable membrane is also affixed to the perimeter walls and is loosely laid over the flexible buoyant layer. In this aspect of the invention, the membrane cover has a substantial thickness. Each drain has a straight pipe extending above the upper surface of the membrane cover for accumulating a level of rainwater in each trough and for maintaining a water connection between the troughs. The drains are located in a one-quarter portion of the width of the cover nearest one of the perimeter walls.
The water ballast contained in the troughs is advantageous for its expanse and shape. The water ballast covers a major area of the membrane cover without having large cross dimension exposed to up-lifting wind forces. Because of the drains, the water ballast remains present on the impermeable membrane for extended period of time following a rainstorm.
In a further aspect of the present invention, the thickness of the membrane cover cooperates with the drains to maintain the level of the water ballast higher than the level of wastewater inside the reservoir, for draining rainwater in excess of the effective level of the water ballast.
Another advantage of the structure of the membrane cover is that the flexible buoyant layer, the impermeable membrane and the array of weight lines constitute three separate layers that are loosely laid over each other. These three separate layers are therefore free to slide upon each other and flex to follow the movement of the wastewater inside the reservoir without generating any destructive tangential stress in the membrane cover.
In another feature of the present invention the membrane cover further has an array of ridges formed thereon each of which being disposed between two of the lateral weight lines. These ridges are advantageous for offering gas passages to evacuate off-gases that are often generated by the wastewater inside the reservoir.
Other advantages and novel features of the present invention will become apparent from the following detailed description.